This invention relates to a novel method of making a quadrupolar focusing color-selection structure for a CRT (cathode-ray tube).
A commercial shadow-mask-type color television picture tube, which is a CRT, comprises generally an evacuated envelope having therein a target comprising an array of phosphor elements of three different emission colors arranged in color groups in cyclic order, means for producing three convergent electron beams directed towards the target, and a color-selection structure including a masking plate between the target and the beam-producing means. The masking plate shadows the target, and the differences in convergence angles permit the transmitted portions of each beam, or beamlets, to select and excite phosphor elements of the desired emission color. At about the center of the color-selection structure, the masking plate of a commercial CRT intercepts all but about 18% of the beam current; that is, the plate is said to have a transmission of about 18%. Thus, the area of the apertures of the plate is about 18% of the area of the masking plate. Since there are no focusing fields present, a corresponding portion of the target is excited by the beamlets of each electron beam.
Several methods have been suggested for increasing the transmission of the masking plate; that is, increasing the area of the apertures relative to the area of the masking plate, without substantially increasing the excited portions of the target area. In one approach, each of the apertures of the color-selection structure is defined by a quadrupolar electrostatic lens which focuses the beamlets passing through the lens in one direction and defocuses them in another direction on the target depending upon the relative magnitudes and polarities of the electrostatic fields comprising the lens. In one type of quadrupolar-lens color-selection structure described, for example, in U.S. Pat. No. 4,059,781 to W. M. Van Alphen et al, a strong focusing quadrupolar lens is generated from voltages applied between an apertured masking plate and an array of conducting strips which are disposed between columns of the apertures and are insulatingly spaced from one major surface of the plate. In a typical color-selection structure of this type, the apertures may be about 0.65 mm (26 mils) wide on about 0.77 mm (30 mils) centers horizontally and 0.62 mm (24 mils) high on about 0.77 mm (30 mils) centers vertically, and the conducting strips may be about 0.14 mm (6 mils) wide and spaced about 0.05 mm (2 mils) from the plate.
Because of the small and precise sizes required of the apertures and the strips, special techniques must be employed to fabricate structures of this type at reasonable cost. Several methods have been suggested previously. But, each prior method appears to be too costly and may not produce an adequate yield of acceptable structures.